Have it interview you
Also called Interview me
Instead of writing the brief yourself, you ask the agent to question you until it could write it. It finds the decisions you hadn't noticed you were making.
You can't list what you haven't thought of. A model has read a great many specs for this kind of feature, so it is good at what must be decided and knows nothing about your answers. That split is the useful part: it brings the list, you bring the choices.
Say it to your agent
“I want to add rate limiting to the public API but haven't thought it through yet. Interview me until you could write the spec, then write it to SPEC.md.”